Drill steel for percussive drilling devices

ABSTRACT

A drill string for use with percussive type drills for rock drilling is described. The drill string has sections of drill steel in which a steel core, which acts as a transmission line for the force pulses generated by the hammer of the drill, is protected from scoring (as from rocks or adjacent metal objects) by sheathing which is attached to the core by resilient members to define a mass spring system in which the sheathing and pulse transmission member are dynamically isolated from each other at the repetition frequency of the force pulses. The sections are connected by coupling joints of the drill string. A breakout mechanism may be incorporated into each section to break out the joints, in which lugs extending from the core engage the sheathing during breakout to protect the resilient member during breakout. The resilient members are constructed so as to be provided with sufficient longitudinal and torsional stiffness such that during normal drilling the lugs are isolated from the sheathing and do not produce galling or other damage.

United States Patent 1 Bouyoucos 1 Dec. 16, 1975 DRILL STEEL FOR PERCUSSIVE DRILLING [73] Assignee: Hydroacoustics, lnc., Rochester,

22 Filed: June 10, 1974 211 Appl. No.: 477,738

[52] US. Cl. 173/80; 173/DIG. 2; 175/320; 181/33 A [51] Int. Cl. E21C 7/00; E21B 17/00 [58] Field of Search 173/80, 131, 102; 175/320; 267/137; 64/11; 181/33 A Primary Examiner--James A. Leppink Attorney, Agent, or FirmMartin LuKacher [5 7 ABSTRACT A drill string for use with percussive type drills for rock drilling is described. The drill string has sections of drill steel in which a steel core, which acts as a transmission line for the force pulses generated by the hammer of the drill, is protected from scoring (as from rocks or adjacent metal objects) by sheathing which is attached to the core by resilient members to define a mass spring system in which the sheathing and pulse transmission member are dynamically isolated from each other at the repetition frequency of the force pulses. The sections are connected by coupling joints of the drill string. A breakout mechanism may be incorporated into each section to break out the joints, in which lugs extending from the core engage the sheathing during breakout to protect the resilient member during breakout. The resilient members are constructed so as to be provided with sufficient longitudinal and torsional stiffness such that during normal drilling the lugs are isolated from the sheathing and do not produce galling or other damage.

29 Claims, 9 Drawing Figures US. Patent Dec. 16, 1975 Sheet10f2 3,926,265

U.S. Patent Dec.16,1975 Sheet20f2 3,926,265

DRILL STEEL FOR PERCUSSIVE DRILLING DEVICES The present invention relates to an improved drill steel for use with a percussive or impact type drilling device and to drill strings which may be made up of successive sections of such drill steel. By the term drill steel as used herein is meant rods or pipe which transmit forces for drilling or boring to an earth formation.

The drill string provided by this invention is suitable for all types of percussive drills, whether pneumatic or hydraulic, and also to those rigs in which the percussive forces are transmitted to a drill bit with rotation.

It has been found that when drill steel is extended into an earth formation, the steel is subjected to scoring as by rocks in the formation or even by metal parts of the drill rig which guide the steel. Scoring may occur in the presence of side loads such as are typically encountered in drilling. Such side loads occur when the surface of the steel is traveling at finite velocity that occur from rotary or longitudinal motions of the steel as are developed by percussive events, or translations (e.g., retracting the steel from the hole) with or without rotation.

Scoring can be a significant source of failures of drill steels for transmitting percussive force pulses. It is presently believed that such failures are a function of the repetitive frequency of the stress cycles (compressive followed by tensile stress during each cycle) which cause cracks in the steel to grow and cause the steel to fail. More specifically the phenomenon of scoring involves the generation of transient increases in temperature which are localized in the steel. Cracks are formed in the surface of the steel where the temperature gradient develops. Surrounding these cracks, regions of trapped tensile stresses may result. When percussive forces are repetitively applied to the steel, the resulting stress cycles cause these cracks to grow and ultimately the steel fails. Thus the failure is a function of the repetitive percussive forces. Means for protection against scoring in the presence of such forces has heretofore not been available. Existing double-walled drill pipes do not provide such protection (e.g., see US. Pat. No. 3,664,441 nor do either drill pipe centering devices (e.g., see U.S. Pat. No. 2,973,966), or reaction decouplers (see US. Pat. No. 3,797,536). It is a principal feature of this invention to provide drill steels affording such protection, thus achieving longer, reliable operation of drill steels for use in percussive drilling applica-' tions. This protection becomes increasingly important as drilling power levels are increased to provide improved, faster drilling rates.

Accordingly, it is an object of this invention to provide improved drill steels.

It is another object of this invention to provide improved drill strings especially adapted for use with drill rigs utilizing percussive force generators to aid in the drilling.

It is a further object of this invention to provide improved drill steels which afford protection against failure of the drill string from scoring.

It is a still further object of this invention to provide an improved multi-section drill steel having peripheral sheathing in which the sections may be separated or broken out without damage to the sheathing.

It is a still further object of this invention to provide improved drill steels which are effective in reducing the level of noise emanating from percussive type drill rigs in which they can be used.

It is a still further object of this invention to provide improved drill steels which facilitate the drilling of straighter holes.

It is a still further object of this invention to provide improved drill steels which permit a close fit within the hole so as to minimize the air requirements necessary for chip removal.

Briefly described, drill steel embodying the invention which may be included as a section of a drill string, includes a force pulse transmission member for conveying force pulses from, a percussive tool to a load. A sheathing, which may be in the form of a tough steel sleeve, is mounted on the transmission member by a resilient member in a manner such that the sheathing is isolated dynamically from the force pulse transmission member. The sheathing may be arranged in segments separated from each other along the drill string.

The percussive tool provides repetitive impacts, as on a shank which is connected to the force pulse transmission member and which in turn applies the percussive forces to a drill bit, with or without rotation. The sheathing is so sized (viz., is provided with mass) in relation to the compliance presented by the resilient members attached between the drill steel and the sheathing, that the sheathing and the resilient member exhibit a mass and compliance (their mass-spring characteristic) such as to place their resonant frequency below the force pulse repetition frequency. Thus, the sheath is dynamically isolated from the force pulse transmission member so that the sheathing does not follow the cyclical movements of the force pulse trans mission member. Hence, when the force pulse transmission member is driven, the cyclical stresses in the sheathing are substantially below the corresponding stresses in the pulse transmission member. The force pulses are also decoupled from the wall of the hole; thus increasing the drilling forces at the bit. The sheathing affords protection of the force pulse transmission member against abrasion and scoring. Since the axial motion of the sheath is substantially less than the motion of the force pulse transmission member, the sheath is subject to reduced abrasion effects.

A mechanism may be provided also, according to the invention, to allow torque for breakout purposes to be applied to the force pulse transmission member rather than to the exterior sheathing. Lugs may project from the transmission member through openings in the sheathing to enable torque to be applied directly or via the sheathing to that member for breakout. The rotation of the sheathing is limited, as by the lugs, thus protecting the resilient member from damage. The resilient member has sufficient stiffness such that during normal drilling the displacement of the sheathing is limited and it does not contact the lugs.

The drill string may be comprised of a plurality of drill steel sections each having a sheathing separate from its force pulse transmission member. The force pulse transmission members are connected directly to each other by being screwed together as through a coupling. The sheathing for each section does not extend the full longitudinal length of that section and need not be connected to the sheathing of the next section or to the coupling, since the sheathing is dynamically isolated and does not interfere with the transmission of force pulses.

The foregoing, other and additional objects advantages and features of the invention will become more readily apparent from a reading of the following specification in connection with the accompanying drawings in which:

FIG. I is a longitudinal section view of a drill string embodying the invention;

FIG. 2 is a cross sectional view of the steel shown in FIG. 1, the section taken along the line 22 in FIG. 1;

FIG. 3 is a longitudinal sectional view of a drill string pipe, a coupling or joint and breakout mechanisms of different types;

FIG. 4 is a cross sectional view of the steel shown in FIG. 3, the section being taken along the line 4-4 in FIG. 3;

FIG. 5 is a fragmentary sectional view of the end of the section of drill steel illustrating another embodiment of a breakout mechanism in accordance with the invention;

FIG. 6 is a fragementary sectional view of the end of a section of drill steel illustrating still another embodiment of a breakout mechanism in accordance with the invention;

FIG. 7 is a fragmentary sectional view of the end of a section of drill steel embodying the invention having the capability of using the chamber between the force pulse transmission member and the sheathing as a conduit for air or fluid through the drill steel;

FIG. 8 is a cross-sectional view of the steel shown in FIG. 7, the section being taken along the line 88 in FIG. 7 and;

FIG. 9 is a plan view of a drilling rig using a drill string embodying the invention.

In FIG. 9 a drilling rig, 10, is shown, having a drill string 11 which is adapted to be connected to tool 12. The'tool 'may have a mechanism for rotating the drill string 11. This tool 12 may be a hydraulically driven hammer or a pneumatically driven hammer. Reference may be had to US. Pat. Nos. 3,382,932 and 3,640,351

for further information respecting such hammers and mechanisms for rotating the drill string 11.

A drill bit 14 is secured to the end of the drill string 11 and is shown in operative position for drilling holes in an earth formation. The hammer, drill string and bit may be carried by a tractor 16. The tractor may also carry apump for hydraulic fluid or a compressor which provides compressed air. The pump or compressor may, however, be carried on an associated vehicle. The drill string 11 and the hammer 12 are mounted on a boom which extends from the tractor 16. The entire apparatus constitutes a drilling rig. Y

The drill string 11 is made up of successive sections 18 of drill steel which are assembled together by couplings or joints 20.

One section 22 of the drill steel which is provided in accordance with the invention is illustrated in FIG. 1. A portion of a succeedingsection 23, which is part of the coupling or joint 25, which connects these sections 22 and 23,-isalso illustrated ,in FIG. I.

The drill steel section 22. has a core member 24 which provides .a force .pulse transmission line for the force pulses generated by the; hammer 12 (FIG. 9).

This core 24 is made out of hexagonal steeLCircular or pins are provided with threads 28 which are engaged by I the box section 30 of the joints 25.

Sheathing 32 in the form of a tubular member or sleeve of a diameter greater than the hexagonal core, 24 is disposed around the core, to form a chamber 34 defined between the core 24 and the sheathing 32. The sheating 32 extends partially over the flared regions at the end of the core 34 and thus is out of contact with both the pin and the box section 26 and 30 of the coupling 25. I

A resilient material is disposed in the chamber 34 and defines a body of resilient material by means of which the sheathing 32 is attached to the core 24. The resilient material of the body 35, is preferably an elastomer which is poured or injected into the chamber 34 and allowed to cure in place. The surfaces of the sheathing 32 and core 24 which contact the body 35 are desirably treated so as to provide for bonding between the elasto mer of the body 35, the sheathing and the core.

The core 24 has a central hole or passage 36 which extends also through the pin section 26. Air or other cleansing fluid may be passed through this passage 36 for hole cleaning purposes. Reference may be had to the above cited US. Pat. No. 3,640,351 for further infonnation respecting the design and use of such passages.

The box section 30 of the joint 25 is provided with longitudinal grooves 38. A breakout wrench may be engaged with these grooves for the purpose of assembling and disassembling the sections 22 and 23 of the drill steel as drilling proceeds to greater depths into the formation or as the drill string 1 l is withdrawn from the hole (see FIG. 10). The sheathing 22 is also provided with similar longitudinal slots 40 for breakout wrench engagement. Inasmuch as the core 24 may be hexagonal in shape and since the elastomer body 35 fills the chamber 34, and is bonded to the walls, the torque may be transferred to the core for breakout purposes without damaging the elastomer body 35. Alternatively, the lower and upper ends of the sheathing may be swaged inwardly to conform to the hexagonal core shape, but to be spaced from the core 24, in a manner similar to that shown in FIG. 3. The elastomer 35 is preferably absent in the swaged region. When the wrench engages the sheathing, either in the swaged region or above that region as in the grooves in the sheathing 32, there will be engagement of the swaged region with the flats of the hexagonal core, thus providing for torque transfer through the sheathing to the core from the wrench.

In the operation of the drill string, forcepulses are repetitively transmitted along the transmission line provided by the core member 24. These force pulses cause cycles of compressive and tensile stresses in the core member. Without the sheathing 32, any scoring of the core member, as by rocks in the hole in the formation or by steel guiding members, is likely to giverise to cracks in the core member and result eventually in the failing thereof. The sheathing 32 is dynamically iso lated from the core member. Such isolation is provided by the mass spring characteristics of the system made up of the sheathing and the resilient body 35. The mass (weight) of the sheathing and the compliance of the resilient body 35 defines a mass spring system having a resonant frequency which is different from and preferably below the repetition frequency of the force pulses which are transmitted along the core member 24. Accordingly, the dynamics of the system provides for a longitudinal dynamic motion of the sheathing which is a small fraction of the dynamic motion of the core in response to the force pulses which are transmitted therethrough. Since the ends of the sheathing are spaced from the core 24 and the coupling box section 30, the sheathing will not impact the core or box section thus preventing end damage to the sheathing.

Although the sheathing is subject to bending stresses during rotation as, for example, in a crooked hole, at normal rotation frequencies the number of rotational cycles per unit of time is several orders of magnitude lower than the number of percussive force pulse induced cycles per unit of time. For example, one million stress cycles at the rotation frequency may take between 100 and 200 operating hours whereas one million stress cycles in the longitudinal direction at frequencies of about 6000 force pulses per minute may take only 2 to 3 hours. Fatigue due to repetitive stress cycles is therefore more likely to occur from the longitudinal cycling rather than the rotational cycling. Since the sheath is protected from the longitudinal induced stresses, its life is significantly enhanced.

Accordingly, the reliability and operational life of drill steels in accordance with the invention are improved.

Appropriate design parameters for the sheathing 32 and the resilient body 35 for a 3 inch O.D. member, feet long, may be that the mass of the sheathing 32 could be in the range of 80 pounds while the compliance of the body 35 should be in the range of 17,000 pounds per inch. These parameters will provide a resonant frequency of about Hz which is below the repetition frequency of 75 Hz whhich is generated by certain high performance percussive hammers. In addition, the sheathing 32 and elastomer body 35 provide for sound isolation. Noise generated in the course of force pulse transmission or from the bit region is suppressed by the sound isolation structure provided by the sheathing and elastomer body. Further, the section modulus of the sheathing is preferably sufficiently high to reduce bending of the drill steel and thus is effective in supporting the core in the presence of high pulldown forces which may produce bending. This feature aids in the drilling of straight holes.

Referring to FIG. 3 there is shown a section 40 of sheathed drill steel in accordance with another embodiment of the invention. FIG. 3 also depicts alternative breakout mechanisms 42 and 44 at the opposite end of the drill steel section 40. A coupling or joint 46 which is sheathed to protect the coupling from scoring failures is also illustrated. In the drill steel section 40, a core 48 which may be a cylindrical rod made of steel provides a force pulse transmission line or member. A threaded pin section 50 provided at the end of the core 48 (only one end being shown in detail to simplify the illustration) is assembled to the pin section 52 of a successive drill steel section 54 by the coupling 46. A central passage 56 is provided through the core 48 for air or other cleansing fluid. (See FIG. 4).

A sheathing 58 in the form of a generally cylindrical tube of steel is disposed around and radially spaced from the core 48. The opposite end of the sheathing 58 may be swaged down towards the core 48 in the course of assembly. Nevertheless, clearance 60 (see FIG. 4) is provided between the core and the sheathing, even at the end of the sheathing so as to facilitate relative movement between the core 48 and the sheathing 58. A plurality of annuluses 62 of resilient material which is preferably an elastomer are disposed in the chamber 64 between the outer periphery of the core 48 and the inner periphery of the sheathing 58. These annuluses may be formed by injecting elastomeric material into successive regions of the chamber 64 which are longitudinally spaced from each other and allowing the material to cure in place. Alternatively, the annuluses 62 may be bonded to the inner periphery of the sheathing 58 prior to the insertion of the core 48 into the sheathing. Alternatively, the annuluses 62 may be toroidal or hollow cylindrical bodies of elastomeric material which are preformed. Each annulus may be compressed much in the same manner as a piston ring and inserted in place in the chamber 64.

The upper end 42 of the sheathing 58 is formed with a plurality, say four, longitudinal slots through which four lugs 72 individually extend when the ends of the sheathing are swaged down toward the core 48. The slots are sufficiently wide so as to provide clearance on all sides of the lugs such that the lugs do not engage the sheathing during normal drilling operations.

The opposite or lower end of the core 48 in FIG. 3 shows an alternate configuration. In this instance, the core 48 has four lugs 74 which extend through rectangular holes 76 in the sheathing 58 (see also FIG. 4). Sufficient clearance is provided between the lugs and the walls of the hole 76, such that the lugs remain out of contact with the sheathing during normal drilling operations. The couplings 46 at the opposite ends of the section 40 cooperate with the arrangement of lugs and slots in providing the breakout mechanisms 42 and 44.

The couplings 46 are similar. In addition to the pin sections 50 and 52, the couplings 46 have a box section 78. A cylindrical sleeve 80 of inner diameter greater than the outer diameter of the box section 78 is shown separated from the box section 78 by a plurality of annuluses 82 in the form of rings of resilient material. The material may be an elastomer of the same type used in providing the annuluses 62 and may be assembled in a cylindrical chamber 84 between the sleeve 80 and the box section 78 in the same manner described above in connection with the annuluses 62. Alternatively, the central section of the chamber 84 may be filled with a body of resilient, say elastomeric material, as was discussed above in connection with the elastomer body 35 shown in FIG. 1. The sleeve 80 has a plurality of holes, say four holes 86, ninety degrees apart at the upper end of the sleeve 80 and a similar arrangement of four holes 88 near the bottom of the sleeve, screws which are screwed into the box section 78 extend into these holes 86 and 88; there being sufficient clearance between the heads of the screws 90 and the walls of the holes. These screws 90 operate in the same manner as the lugs 72 and 74. A plurality of longitudinal grooves 92 for engagement with a breakout wrench are provided in the outer surface of the sleeve 80. The arrangement of these grooves may be similar to the arrangement of the grooves 38 (FIG. 1). Similar arrangements of grooves 94 and 96 are also provided in the sheathing 58.

The tough steel sheathing 58 and the annuluses 62 of resilient material provide for dynamic isolation of the sheathing 58 from the core or force pulse transmission line 48. In other words, the longitudinal dynamic motion of the sheathing 58 is a small fraction of the dynamic motion of the core 48 in transmitting force pulses therethrough at the repetition frequency of these force pulses. The resonant frequency of the mass spring system defined by the mass of the sheathing 58 and the compliance of the annuluses 62 is different from the force pulse repetition frequency and preferably well below the force pulse repetition frequency. Accordingly, during normal drilling, the clearance in a longitudinal direction between the lugs 74 and the slots, or in case the breakout arrangement 42 is used between the lugs 72 and the slots 70, is sufficient such that the lugs do not engage or rub on the walls of the slots. Galling or other damage to the core 48 is therefore prevented.

Also during normal drilling the torsional stiffness of the annuluses 62 is sufficiently high such that rotational torque transferred to the sheathing is not large enough to cause contact of the slots 70 or 76 with the lugs, even though the drill steel is rotated. The sleeve 80 and its rings 82 also defines a system which dynamically isolates the sleeve from the box section 78 of the coupling 46. Sufficient clearance is also provided between the walls of the holes 86 and the projecting heads of the screws 90 such that engagement therebetween is minimized during normal drilling operations. Should however the sheath be displaced by an obstruction either during drilling or on retracting the drill steel from the hole, the lugs 72 or 74 and the screws 90 provide protective stops which prevents excessive force from being transferred to the annuluses 62 or the rings 82 which might cause damage thereto.

During breakout operations however breakout torque is transferred from the wrench directly to the box section 78 of the coupling by way of the screws 90 and through the sheathing 58 through the lugs 72 or 74 to the core 48. Thus, the considerable torque which is needed in breakout can be applied without damaging the resilient material of the annuluses 62 or the rings 82 which afford isolation of the sheathing and sleeve during normal drilling operations.

As shown in FIG. 4 the lugs preferably do not extend beyond the sheathing 58 and are disposed well within the slot 76. The possibility of the lugs engaging any obstacle in the hole is therefore reduced.

Consider by way of example the exemplary case where force pulses which are transmitted along the force pulse transmission line provided by the core 48 have a repetition frequency of 100 Hz and where the diameter of the core is 2 inches. In such case the annuluses 62 may be an elastomer material such as natural rubber, each having a longitudinal shear stiffness of 3400 pounds per inch. The sheathing is of tough steel having a mass of 80 pounds for a sheathing length of 20 feet and diameter of 3 inches. With the use of five annuluses, each one inch in length the resonant frequency of the sheathing is about 40 Hz. The number of annuluses 62, the compliance of the material used therefor, and the mass of the sheathing will vary in accordance with the pulse repetition frequency and the length of the drill steel section 40 and may be selected to provide the requisite resonant frequency below the frequency of repetition of the force pulses for dynamically isolating the sheathing 58 from the core 48. Similar considerations may be used in selecting the sleeve 92 and rings 82 of the joint 46.

The sheathed drill steel shown in FIGS. 3 and 4 also has the features discussed in connection with the steel shown in FIG. 1 of drilling straighter holes by virtue of the restraint on bending of the core 48 imposed by the sheathing 58. The sleeve 80 around the joint 46 as well as the sheathing 58 together with their rings 82 and annuluses 62 also provide acoustic isolation which affords muffling of noises generated in percussive drilling operations.

FIG. 5 illustrates another sheathed drill steel in accordance with the invention having a force pulse transmitting core and a tubular sheathing 102 assembled on the core 100 by means of annuluses 104 of resilient material, as was discussed in connection with FIG. 3. A threaded pin 106 is shown for coupling purposes. The breakout mechanism may include a joint similar to the joint 46 and pins or rods 108 which are assembled as by being force fit into holes in the core 100. The length of the pin is greater than the diameter of the core 100 but less than the outer diameter of the sheathing 102. The pin extends into holes 110 which are diametrically opposite each other in the sheathing 100. Two pins are shown, one above the other, and the axes of the pins are displaced 90 with respect to each other. Clearance is provided between the walls of the holes 110 and the pins 108. Longitudinal grooves 112 are also provided for breakout wrench engagement purposes. The operation of the pin is similar to that described for the lugs 72 and 74 as discussed above in connection with FIG. 3.

In FIG. 6 there is shown a force pulse transmission core member 114 which is formed at its end with flats 116, four or six of which may be provided. The flats are arranged much like a nut and may be engaged by a wrench for breakout purposes.

A sheathing 118 around the core 114 is separated from the core by annuluses 120 for dynamic isolation purposes as was discussed in connection with FIGS. 1 and 3. Another annulus 122 is disposed between the end section of the core which is formed with the flat 1 l6 and the lower end of the sheathing 118. This annulus 122 extends outwardly so as to provide a bumper for protecting the lower end of the sheathing. Instead of the flats l 16, the section of the core 114 may also be formed with serrations for breakout wrench engagement purposes. The sheathed steel embodiment as shown in FIGS. 5 and 6 may also be completely filled with resilient material or the annuluses 120, as shown, may be used.

Referring to FIGS. 7 and 8, there is shown a section of drill steel 124 and a joint 126. The section 124 is a sheathed steel section having a force pulse transmission core 128 separated from a tubular sheathing 130 by several longitudinally spaced annuluses 132 of resilient material. The bottom one, 134 of these annuluses has a lip which extends between the box 136 of the coupling 126. The box 126 engages the threaded pin 138 at the end of the core 128 and assembles it to the pin 140 of the next core section. The annulus 134 and the upper end of the box 126 have aligned axial holes 140, several of which are spaced from each other along the circumference of a circular pin (see FIG. 8). The holes in the upper end of the box 126 may be relieved or a groove between these holes may be used to provide tolerance in alignment. Similar arrangements of holes 142 are provided in the annuluses 132. Cleansing fluid such as compressed air may flow through these holes and to the chambers 144 and 146 in the sheathed steel section 124 and the joint 126. This cleansing fluid may be used for purposes of hole cleaning and removal of chips at the bottom of the hole. The arrangement of resilient annuluses 132 and 134 and sheathing 130 provide for dynamic isolation of the core as was explained above in connection with FIGS. 1 and 3.

From the foregoing description it will be apparent that there has been provided improved drill steel having a greater reliability than steels heretofore provided and which are especially suitable for use in drill steels which are operative to transmit force pulses at high force pulse repetition rates. While various embodiments of drill steel in accordance with the invention have been described above, variations and modifications in the herein described embodiment within the scope of the invention will undoubtedly suggest themselves to those skilled in the art. Accordingly, the foregoing description should be taken as illustrative and not in any limiting sense.

What is claimed is:

1. Drill steel for use with percussive tools which generate force pulses at a certain repetition frequency comprising:

a. a force pulse transmission member,

b. a sheathing surrounding said member, said sheathing being spaced from said transmission member to define a chamber separating said member from said sheathing, and

c. means in said chamber connecting said sheathing and said transmission member to cause said sheathing to have a resonant frequency for motion of said sheathing in the direction of force pulse transmission along said transmission member which resonant frequency is below the force pulse transmission frequency whereby said sheathing is dynamically isolated from said transmission member, said means comprising a resilient member of elastic material located in said chamber and connecting said sheathing and said transmission member, said sheathing having a certain mass and said resilient member having a certain compliance related to the mass of said sheathing together to constitute a mass spring system which is resonant at said'resonant frequency.

2. The invention as set forth in claim 1 wherein said force pulse transmission member is a core and said sheathing is a tubular member surrounding said transmission member.

3. The invention as set forth in claim 2 wherein said transmission member and said sheathing are both constructed of steel.

4. The invention as set forth in claim 2 wherein said resilient member is constituted of elastomeric material.

5. The invention as set forth in claim 4 wherein said material substantially fills said chamber.

6. The invention as set forth in claim 4 wherein said resilient member includes a plurality of annuluses of said material, spaced from each other in a longitudinal direction, along said core.

7. The invention as set forth in claim 6 wherein said annuluses each have a plurality of openings extending axially therebetween and providing channels for the flow of cleansing fluid through said chamber.

8. The invention as set forth in claim 4 wherein said elastomeric material is bonded at least to one of said sheathing and said core.

9. The invention as 'set forth in claim 4 wherein said core is a rod having planar longitudinal faces.

10. The invention as set forth in claim 9 wherein said faces define a hexagon in a plane normal to the axis of the rod.

11. The invention as set forth in claim 2 including a joint adapted to be secured at the end of said core, said coupling joint having a coupling member, said coupling member and sheathing being spaced from each other.

12. The invention as set forth in claim 2 further comprising a joint for interconnecting successive sections of said drill steel into a drill string, said joint including a coupling member for engaging the ends of said successive sections, a sleeve around said coupling member, and a resilient member disposed between said sleeve and coupling member, holding said sleeve and coupling member in assembled relationship and dynamically isolating said sleeve from said coupling member at said force pulse repetition frequency.

13. The invention as set forth in claim 12 wherein said sleeve is spaced longitudinally from said sheathing so as to be out of contact with each other.

14. The invention as set forth in claim 12 wherein said sleeve and said sheathing are of substantially the same outside diameter.

15. The invention as set forth in claim 12 wherein said resilient member which is disposed between said coupling member and its sleeve is comprised of a plurality of rings of elastomeric material spaced from eachother in a direction longitudinally of said coupling member.

16. The invention as set forth in claim 12 wherein said sleeve around said coupling member has first and second pluralities of holes, said first plurality of holes being spaced around said sleeve near one end thereof, said second plurality of holes being spaced around said coupling member sleeve near the end thereof opposite to said one end, and lug means extending laterally from said coupling member sleeve into said holes, said lug means being smaller than the holes through which they extend so that clearance is provided between said coupling member sleeve and said lug means.

17. The invention as set forth in claim 16 wherein said lug means are provided by bolts extending radially into said coupling member threadly engaging said coupling member.

18. The invention as set forth in claim 2 wherein said core is polyogonal in cross section and the said tubular sheathing member at the opposite ends thereof is swaged inwardly toward said core.

19. A drill steel for use with a percussive tool, said steel comprising:

a. an elongated core member for transmitting force pulses generated by said tool,

b. a tubular sheathing disposed around and radially spaced from said core member, said sheathing extending substantially the entire length of said core member,

c. an annular member of elastomeric material disposed between said sheathing and said core member, said annular member holding said sheathing spaced from said core member and in assembled relationship therewith, and

cl. breakout means for separating said steel from a coupling member, said breakout means including members projecting from said core member and spaced from said sheathing.

20. The invention as set forth in claim 19 wherein said breakout means includes a plurality of said projecting members, said sheathing having openings through which said projecting members extend, said openings being larger than said projecting members to provide clearance therebetween.

21. The invention as set forth in claim 20 wherein said openings are slots and said projecting members are lugs.

22. The invention as set forth in claim 21 wherein said slots extend longitudinally from the ends of said sheathing.

23. The invention as set forth in claim 21 wherein said lugs are rectangular in cross section and said openings are rectangular, having sides which correspond to the sides of said lugs, each side of said openings being larger than a corresponding side of the lug which projects therein.

24. The invention as set forth in claim wherein the ends of said sheathing are flared inwardly toward said core, said opening being in said flared ends, and said projecting members having a length less than the diameter of said flared ends.

25. The invention as set forth in claim 20 wherein said projecting members are provided by a-pin extending laterally through said core.

26. The invention as set forth in claim 25 wherein a plurality of said pins are provided each disposed in a separate plane space axially of said core, the axis of said pins being transverse to each other.

27. The invention as set forth in claim 19 wherein said members of said breakout means consist of a portion of said core at one end thereof, said portion having surfaces forming a nut, said sheathing being of length less than said core and terminating before reaching said nut.

28. The invention as set forth in claim 27, wherein said annular member has a lip extending between said nut and said sheathing to provide a bumper therebetween. 7

29. The invention as set forth in claim 19 further comprising a joint having a coupling member for securing the cores of successive sections of said drill steel to form a drill string, said annular member comprises a plurality of annuluses longitudinally spaced from each other along said core member and wherein one of said plurality of annuluses is disposed at the end of said sheathing, said one annulus having a lip extending radially outward to form a bumper between said coupling member and the end of said sheathing. 

1. Drill steel for use with percussive tools which generate force pulses at a certain repetition frequency comprising: a. a force pulse transmission member, b. a sheathing surrounding said member, said sheathing being spaced from said transmission member to define a chamber separating said member from said sheathing, and c. means in said chamber connecting said sheathing and said transmission member to cause said sheathing to have a resonant frequency for motion of said sheathing in the direction of force pulse transmission along said transmission member which resonant frequency is below the force pulse transmission frequency whereby said sheathing is dynamically isolated from said transmission member, said means comprising a resilient member of elastic material located in said chamber and connecting said sheathing and said transmission member, said sheathing having a certain mass and said resilient member having a certain compliance related to the mass of said sheathing together to constitute a mass spring system which is resonant at said resonant frequency.
 2. The invention as set forth in claim 1 wherein said force pulse transmission member is a core and said sheathing is a tubular member surrounding said transmission member.
 3. The invention as set forth in claim 2 wherein said transmission member and said sheathing are both constructed of steel.
 4. The invention as set forth in claim 2 wherein said resilient memBer is constituted of elastomeric material.
 5. The invention as set forth in claim 4 wherein said material substantially fills said chamber.
 6. The invention as set forth in claim 4 wherein said resilient member includes a plurality of annuluses of said material, spaced from each other in a longitudinal direction, along said core.
 7. The invention as set forth in claim 6 wherein said annuluses each have a plurality of openings extending axially therebetween and providing channels for the flow of cleansing fluid through said chamber.
 8. The invention as set forth in claim 4 wherein said elastomeric material is bonded at least to one of said sheathing and said core.
 9. The invention as set forth in claim 4 wherein said core is a rod having planar longitudinal faces.
 10. The invention as set forth in claim 9 wherein said faces define a hexagon in a plane normal to the axis of the rod.
 11. The invention as set forth in claim 2 including a joint adapted to be secured at the end of said core, said coupling joint having a coupling member, said coupling member and sheathing being spaced from each other.
 12. The invention as set forth in claim 2 further comprising a joint for interconnecting successive sections of said drill steel into a drill string, said joint including a coupling member for engaging the ends of said successive sections, a sleeve around said coupling member, and a resilient member disposed between said sleeve and coupling member, holding said sleeve and coupling member in assembled relationship and dynamically isolating said sleeve from said coupling member at said force pulse repetition frequency.
 13. The invention as set forth in claim 12 wherein said sleeve is spaced longitudinally from said sheathing so as to be out of contact with each other.
 14. The invention as set forth in claim 12 wherein said sleeve and said sheathing are of substantially the same outside diameter.
 15. The invention as set forth in claim 12 wherein said resilient member which is disposed between said coupling member and its sleeve is comprised of a plurality of rings of elastomeric material spaced from each other in a direction longitudinally of said coupling member.
 16. The invention as set forth in claim 12 wherein said sleeve around said coupling member has first and second pluralities of holes, said first plurality of holes being spaced around said sleeve near one end thereof, said second plurality of holes being spaced around said coupling member sleeve near the end thereof opposite to said one end, and lug means extending laterally from said coupling member sleeve into said holes, said lug means being smaller than the holes through which they extend so that clearance is provided between said coupling member sleeve and said lug means.
 17. The invention as set forth in claim 16 wherein said lug means are provided by bolts extending radially into said coupling member threadly engaging said coupling member.
 18. The invention as set forth in claim 2 wherein said core is polyogonal in cross section and the said tubular sheathing member at the opposite ends thereof is swaged inwardly toward said core.
 19. A drill steel for use with a percussive tool, said steel comprising: a. an elongated core member for transmitting force pulses generated by said tool, b. a tubular sheathing disposed around and radially spaced from said core member, said sheathing extending substantially the entire length of said core member, c. an annular member of elastomeric material disposed between said sheathing and said core member, said annular member holding said sheathing spaced from said core member and in assembled relationship therewith, and d. breakout means for separating said steel from a coupling member, said breakout means including members projecting from said core member and spaced from said sheathing.
 20. The invention as set forth in claim 19 wherein said breakout means includes a plurality of said projecting members, sAid sheathing having openings through which said projecting members extend, said openings being larger than said projecting members to provide clearance therebetween.
 21. The invention as set forth in claim 20 wherein said openings are slots and said projecting members are lugs.
 22. The invention as set forth in claim 21 wherein said slots extend longitudinally from the ends of said sheathing.
 23. The invention as set forth in claim 21 wherein said lugs are rectangular in cross section and said openings are rectangular, having sides which correspond to the sides of said lugs, each side of said openings being larger than a corresponding side of the lug which projects therein.
 24. The invention as set forth in claim 20 wherein the ends of said sheathing are flared inwardly toward said core, said opening being in said flared ends, and said projecting members having a length less than the diameter of said flared ends.
 25. The invention as set forth in claim 20 wherein said projecting members are provided by a pin extending laterally through said core.
 26. The invention as set forth in claim 25 wherein a plurality of said pins are provided each disposed in a separate plane space axially of said core, the axis of said pins being transverse to each other.
 27. The invention as set forth in claim 19 wherein said members of said breakout means consist of a portion of said core at one end thereof, said portion having surfaces forming a nut, said sheathing being of length less than said core and terminating before reaching said nut.
 28. The invention as set forth in claim 27, wherein said annular member has a lip extending between said nut and said sheathing to provide a bumper therebetween.
 29. The invention as set forth in claim 19 further comprising a joint having a coupling member for securing the cores of successive sections of said drill steel to form a drill string, said annular member comprises a plurality of annuluses longitudinally spaced from each other along said core member and wherein one of said plurality of annuluses is disposed at the end of said sheathing, said one annulus having a lip extending radially outward to form a bumper between said coupling member and the end of said sheathing. 